Charles Darwin could have saved himself from some of his first critics if he had remembered to include his preface in the first edition of his influential book on evolution and natural selection, "The Origin of Species."

The original edition of the landmark book was published in 1859, without any introductory material. Still a public flash point today, "Origin" drew initial outcries, in part for the missing preface in which an author of the time typically would have credited and thanked his intellectual predecessors. One scientist accused Darwin of plagiarism for failing to acknowledge the giants whose work allowed him to see farther.

A year later, a new edition of the classic was published with a preface describing and crediting the intellectual work upon which natural selection rests.

A new study shows that Darwin was composing the introductory chapter to his book as early as 1856, even though that preface remained unpublished for six years until Origin's second edition. The findings suggest Darwin also wrote the chapter on his own accord and not in reaction to people who had read the book.

Darwin historians have assumed the chapter "Historical Sketch" was penned as an afterthought, in response to his critics and accusations that his ideas were not original. However, Darwin's personal correspondence shows that the sketch was actually written prior to the first printing of the book, said political historian Curtis Johnson of Lewis and Clark College, whose detailed study of the preface is published in the January issue of the Journal of the History of Biology.

"Darwin was not reacting to hostile criticism," Johnson said. "My work shows that he had already written most of the sketch before he wrote 'The Origin of Species.' The question is, why didn't he include it in the original edition?"

During the mid-1800s, the theory of evolution was a hot topic and most people opposed the idea. Darwin's preface charted the history of evolution-related contributors and observations that came before him, similar to introductions for other scientific works of the time.

The first published version of the preface credited Charles' uncle, Erasmus Darwin; French naturalist Jean-Baptiste Lamarck; and 12 others. From the first appearance of the preface in 1860 to the final edition during his lifetime, Darwin made several changes and expanded the list of his intellectual debts to include 36 people.

Why he chose to mention some thinkers and not others remains a mystery.

Glaring omissionOne surprising absence, said Johnson, is geologist Charles Lyell, author of "Principles of Geology." Darwin had that book aboard the HMS Beagle on his trip to the Galapagos and beyond, and most likely read its famous historical introduction. In 1856—almost four years prior to the publication of "The Origin of Species"—Darwin wrote a letter to Lyell describing his work on a preface for his own book.

"In his book, Lyell anticipates evolutionary thought," Johnson told LiveScience. "Yet Darwin didn't put Lyell in his Historical Sketch [preface]. If I would have guessed one person for Darwin to include, I would have guessed Lyell."

Sometime between his letter to Lyell and the first edition of "The Origin of Species," the sketch slipped from Darwin's mind.

An angry letter from English mathematician Baden Powell reminded Darwin of his lapse.

In the letter, dated 1860, Baden accused Darwin of plagiarizing his concepts of evolution. Darwin responded apologetically and explained that he just remembered writing a preface in which he acknowledged Baden's work.